So, the most critical article that I think everyone should read about general agile software development is a somewhat older article from Ars. It covers how Microsoft turned the ship around on one of their flagship products and kept themselves relevant when they could have easily lost their footing in the market.
Secondly, this should be required viewing for anyone doing agile. Even if you’re not a TDD person, the fundamental concepts here are absolutely vital to doing good software development in general.
These two are basically where everyone should start on their agile journey.